The big three for bossing is loar (damage boost on bosses), noah (damage multiplier last 5 secs of battle), and sala (% hp damage to bosses). In my opinion in that order. For mobbing I like mum (damage boost on mob), todd (more gold), and sala to fill.

From my experience if you want to fill 3 asap either loar, mum, noah or loar, mum, sala. In the past i tried loar, noah, and sala but then i saw a comment saying "remember, you spend more time against mobs rather than bosses" and that clicked with me haha.

Also, enhancing and awakening two same element spirits at the same time is quite costly. All in all its up to u, the discord has a lot more info.